An NYPD sergeant who once headed the city’s gay-cops coalition sexually preyed upon a relative’s adopted son, forcing the boy to have sex on a weekly basis, authorities charged yesterday.

Jaime Katz, a 10-year veteran assigned to the Police Academy, “developed a relationship with the orphan, and over time groomed him for sex and sex-related acts,” Assistant DA John Temple said at the cop’s arraignment last night.

Katz pleaded not guilty and is being held in lieu of $500,000 bail.

Prosecutors said the victim “lost both his parents at a very young age.” A source said one of Katz’s relatives then adopted the child and his older brother.

The abuse began when the boy was 11 and continued until he was 13, Temple said. The victim is now 18.

Temple said Katz paid bills for the victim’s adoptive family and went on vacations with them.

The sergeant “utilized various methods to avoid detection and surveillance,” Temple said, including the use of multiple cellphones.

Katz, 37, is charged with endangering the welfare of a child and a criminal sex act.

His defense lawyer, Andrew Quinn, told the judge his client “has known about these allegations for three years and has submitted himself to questioning with [NYPD] Internal Affairs.

“The allegations have no credibility due to the boy’s denial of the sexual act occurring,” Quinn said.

Katz, former head of the Gay Officers Action League, used to lead the group in the Gay Pride Parade.

“I’m gay, very gay,” Katz told the Halifax Daily News in Canada after marching in that city’s 2002 parade.

Katz said he wanted to show gay youths that they don’t have to be ashamed.

“There’s a lot of kids out there who have a tough time being who they are,” he told the paper. “I think [that] to see that you can be anything and be gay and be accepted, that’s a nice thing.”